High-power or high-brightness semiconductor laser sources which have high efficiency are required for a variety of applications including machining, laser pumping and numerous medical procedures. Efficient high brightness semiconductor laser sources are typically achieved by focusing a semiconductor laser beam into an optical fiber having a small etendue (i.e. small product of core diameter and numerical aperture of the fiber).
Prior methods of fiber coupling high-power diode laser arrays, however, require the use of highly multi-mode optical fiber (i.e. large etendue) and, therefore, have relatively low brightness. For example, one commercial product generates 30 Watts of output power from a multi-mode fiber with a core diameter of about 1 mm and a numerical aperture of 0.12.
Numerous other applications require high-power or high-brightness sources. These applications include communications, solid state laser pumping, imaging, printing, and optical switching. Relatively low-power, multi-wavelength integrated and external cavity lasers have been constructed using dispersive elements.
U.S. Pat. No. 5,351,262 to Poguntke et al. describes a multi-wavelength laser having an integrated cavity that is formed on a single substrate. The laser includes a plurality of individually selectable active waveguides, a diffraction grating, and a passive output waveguide. A resonant cavity is formed between the selected active stripe, the diffraction grating, and the passive output waveguide. The geometry of the resonant cavity determines the lasing wavelengths of each of the plurality of active waveguides. The Poguntke laser can only be used to generate relatively low powers because it is integrated on a monolithic substrate and thus has limited heat dissipation.
Farries, et al., Tunable Multiwavelength Semiconductor Laser with Single Fibre Output, Electronic Letters, Vol. 27, No. 17, Aug. 15, 1991, describes a low-power multi-wavelength external cavity laser that uses a diffraction grating. The external cavity comprises a monolithic semiconductor laser array, a diffraction grating, and a single mode fiber loop mirror. The loop mirror includes a 50:50 coupler with two output ports that are fusion spliced to form a Sagnac interferometer.
Because the Farries laser is designed for fiber optic communication systems, it comprises a single mode semiconductor laser array and, therefore, it can only be used to generate relatively low powers. In Farries, the element separation in the semiconductor laser array is only ten microns. The resulting output power into the fiber is only approximately 0.5 mW per element. In addition, because the Farries laser is not completely a free space laser cavity, it is relatively inefficient.
U.S. Pat. No. 5,115,444 to Kirkby et al. describes a multi-wavelength external cavity and integrated cavity laser that uses a dispersive element. A set of optical cavities having different frequency bands is formed from a set of individually addressable semiconductor laser amplifiers, each having a single reflecting facet. The cavity includes a common dispersive element and a common semiconductor amplifier having a partially reflecting facet. The Kirkby laser can only be used to generate relatively low powers. The Kirkby integrated cavity laser is formed on a monolithic substrate and thus has limited heat dissipation. The Kirkby external cavity laser uses a common semiconductor amplifier through which all optical beams in the cavity must propagate. Because the common amplifier also has limited heat dissipation, the Kirkby external cavity laser can only generate relatively low power.
U.S. Pat. No. 5,379,310 to Papen et al. describes an external cavity multi-wavelength laser that uses a dispersive element. A cavity is formed from a plurality of semiconductor lasers, a dispersive element and a reflective element. The plurality of semiconductor lasers generates a plurality of optical beams which are deflected by the dispersive element onto the reflective element. The combination of the dispersive element and the curved surface imposes a different resonance condition on each semiconductor laser thereby resulting in each laser lasing at a different wavelength. The Papen laser generates a plurality of parallel output beams; each beam having a different wavelength. The Papen laser is designed for relatively low power applications such as communication systems, data storage, and spectroscopy. Because the Papen laser generates a parallel (not overlapping or coaxial) output beam, it has relatively low brightness.
U.S. Pat. No. 4,923,270 to Carter describes a laser source for optical wavelength multiplexing. The laser source includes an optical assembly and a reflection grating for collimating light emitted by a laser and for refocussing the light onto an output waveguide. The output waveguide is a fiber that incorporates an embedded reflector which is displaced from the end of the fiber. Because the cavity of the laser source is not a completely free space laser cavity, the power, efficiency, and brightness of the laser is limited.